In modern wireless telecommunication systems, such as in a LTE communication system, there exist methods for determining the position of a mobile device, or a mobile terminal, within the system. The accuracy of these methods varies. Nevertheless, these methods have enabled application developers and wireless network operators to provide position based services. For example, services such as guiding systems, shopping assistance, friend finder, presence services, community and communication services and other information services have been developed. In this manner, the user of the mobile device receives local information based on where the user is positioned, or located.
Moreover, in addition to the commercial services, governments in several countries have put requirements on the network operators to be able to determine the position of a mobile terminal placing an emergency call. The requirements make no difference between indoor and outdoor environment.
In outdoor environments, estimation of the position of the mobile terminal may be performed by using positioning systems, such as GPS (Global Positioning System) based methods like Assisted-GPS (A-GPS). The estimation of the position may also be performed by using the wireless telecommunication system itself. Among the known methods for estimation of the position of the mobile terminal by using the wireless telecommunication system, two main groups may be distinguished.
The first group comprises methods that are based on the radio base station, on which the emergency calling mobile terminal is camping. The estimation of the position is, thus, in this first group, based on Cell-ID or combination of cell-ID and Timing Advance (TA). The TA measurement principle is depicted in FIG. 1. Briefly, the travel time of radio waves from a radio base station 120 to a mobile terminal 170 and back is measured. This travel time is hereinafter referred to as round trip time, RTT. The distance, r (as indicated by the dashed arrow r), from radio base station to mobile terminal is the obtained as:
      r    =          c      ⁢              TA        2              ,where TA is the round trip time and where c is the speed of light. The round trip time measurement alone defines a circle (a portion thereof is shown in FIG. 1 as denoted by 240), or if the inaccuracy is accounted for, a circular strip around the radio base station. By combining this information with the polygons 210, 220, 240 of the radio base station 120, left and right angles of the circular strip may be computed, thereby improving the accuracy of the determined measure of the position of the mobile terminal. Hence, the position of the mobile terminal is determined by calculating the intersection of the serving cell, i.e. area covered by the radio base station on which the mobile terminal is camping, and the circular strip. In several systems, among these the LTE system, TA may be used to identify the distance from the antenna at which a mobile terminal is positioned. This provides a distance, but it is not possible to determine where in a sphere (as defined by distance from the radio base station) or portion of a circular strip the mobile terminal is located. If RTT measurements determine that the mobile terminal is located 500 m from the base station, this is along an arc in a sector or the circumference of a circle.
The second group comprises methods that are based on time of arrival measurements, TOA, from multiple base stations as illustrated in FIG. 2. Expressed in mathematical formulas the time of arrival principle is:tR1=tT1+√{square root over ((x−x1)2+(y−y1)2)}{square root over ((x−x1)2+(y−y1)2)}/c−b+v1  (1a)tR2=tT2+√{square root over ((x−x2)2+(y−y2)2)}{square root over ((x−x2)2+(y−y2)2)}c/+b+v2  (1b)tRN=tTN+√{square root over ((x−xN)2+(y−yN)2)}{square root over ((x−xN)2+(y−yN)2)}c/+b+vN  (1N)where:    tRi: Time of reception for ith base station (measured)    tTi: Time of transmission for ith base station    xb yi: Coordinates of ith base station (known)    c: Speed of light    x, y: Coordinates of MS computed by solving equations (at MS or in network node)    b: Receiver clock bias    vi: Measurement error of ith timing measurement
When solving the equations (1a)-(1N) for the unknowns (x, y, b) with N>=3 and the geographical location of the base stations are appropriate, an estimation of the position of the mobile terminal is obtained in the form of coordinates x and y. One solution is to use numerical optimization solutions based on Taylor series expansions of equations (1a)-(1N). These methods are well known in the art and are, hence, not further elaborated herein.
Estimation of the position of the mobile terminal by using a TOA based method requires that the timing of at least three geographically dispersed base stations is measured. Therefore it is necessary to ensure that the SNR (signal to noise ratio) to said at least three base stations are strong enough so that each base station may be detected by the mobile terminal. Cellular system which reuse the same frequency band are designed to create strong isolation between cells, meaning that the signal from the serving base station should be strong while interference from the neighboring base stations should be minimized. In effect, the requirements for positioning and communication are conflicting. Since LTE is primarily a communication system, time measurements for positioning needs to be done at very low C/I (carrier to interference ratio) to neighboring base stations, which puts high requirements on the mobile terminal receiver and also typically degrades the positioning accuracy.
The timing of dispersed base station may be measured using some of the known signals that are always transmitted from an LTE base station. For example, the timing may be based on synchronization signals or reference signals. In an LTE system, each radio frame comprises 10 subframes.
Synchronization signals are commonly transmitted in subframe 0 and 5. The primary synchronization signal is transmitted in the last OFDM symbol and the secondary synchronization signal is transmitted in the second last OFDM symbol of a subframe. There are 3 different PSS (Primary Synchronization Signal) sequences and 168 different SSS (Secondary Synchronization Signal) sequences. The sequence identities are used to distinguish different base stations (or cells). The identity of the base station can then be used to determine the reference signal sequence and its allocation in the time-frequency grid. The synchronization signals may, for example, occupy 62 resource elements in the centre of an allocated bandwidth.
Reference symbols are transmitted every subframe and over the entire bandwidth. Different base station may use six different shifts in frequency and 504 different signals exist. In practice, there is a reuse 3 pattern for reference symbols (2 TX antennas assumed). In low load, the interference could then be favorable for time measurements on reference symbols. In high load, however, the situation becomes similar to PSS/SSS for synchronization signals.
In U.S. Pat. No. 6,064,888, there is disclosed a method for determining a geographical position of mobile terminal operating in a TDMA/FDMA (GSM) system. Once a position determination is needed, the terminal is instructed to transmit a certain sequence at a certain time slot on a certain frequency, and base stations perform TOA or TDOA (Time Difference of Arrival). Then, the mobile terminal is forced to do an inter frequency HO and the procedure is repeated on that frequency. Once sufficiently many TOA/TDOA estimates are obtained, the position of the mobile terminal may be estimated. This approach introduces a need for the base stations to do the measurements and the mobile terminal to transmit the certain positioning signals.